The technology of the present disclosure relates generally to distributed antenna systems (DASs) for distributing communications services to remote areas each forming a coverage area and particularly to separation of communications signal sub-bands in DASs to reduce interference.
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, local area wireless services (e.g., so-called “wireless fidelity” or “WiFi” systems) and wide area wireless services are being deployed in many different types of areas (e.g., coffee shops, airports, libraries, etc.). Distributed communications or antenna systems communicate with wireless devices called “clients,” “client devices,” or “wireless client devices,” which must reside within the wireless range or “cell coverage area” in order to communicate with an access point device. Distributed antenna systems are particularly useful to be deployed inside buildings or other indoor environments where client devices may not otherwise be able to effectively receive radio-frequency (RF) signals from a source, such as a base station for example. Example applications where distributed antenna systems can be used to provide or enhance coverage for wireless services include public safety, cellular telephony, wireless local access networks (LANs), location tracking, and medical telemetry inside buildings and over campuses.
One approach to deploying a distributed antenna system involves the use of RF antenna coverage areas, also referred to as “antenna coverage areas.” Antenna coverage areas can be formed by remotely distributed antenna units, also referred to as remote units (RUs). The remote units each contain or are configured to couple to one or more antennas configured to support the desired frequency(ies) or polarization to provide the antenna coverage areas. Antenna coverage areas can have a radius in the range from a few meters up to twenty meters as an example. Combining a number of remote units creates an array of antenna coverage areas. Because the antenna coverage areas each cover small areas, there typically may be only a few users (clients) per antenna coverage area. This arrangement generates a uniform high quality signal enabling high throughput supporting the required capacity for the wireless system users.
As an example, FIG. 1 illustrates distribution of communications services to remote coverage areas 10 of a DAS 12. In this regard, the remote coverage areas 10 are created by and centered on remote units 14 connected to a head-end equipment (HEE) 16 (e.g., a head-end controller or head-end unit). The head-end equipment 16 may be communicatively coupled to a base station 18. In this regard, the head-end equipment 16 receives downlink communications signals 20D from the base station 18 to be distributed to the remote units 14. The remote units 14 are configured to receive downlink communications signals 20D from the head-end equipment 16 over a communications medium 22 to be distributed to the remote coverage areas 10 of the remote units 14. Each remote unit 14 may include an RF transmitter/receiver (not shown) and an antenna 24 operably connected to the RF transmitter/receiver to wirelessly distribute the cellular services to client devices 26 within the remote coverage area 10. The remote units 14 are also configured to receive uplink communications signals 20U from the client devices 26 in the remote coverage area 10 to be distributed to the base station 18. The size of a given remote coverage area 10 is determined by the amount of RF power transmitted by the remote unit 14, the receiver sensitivity, antenna gain and the RF environment, as well as by the RF transmitter/receiver sensitivity of the client device 26. Client devices 26 usually have a fixed RF receiver sensitivity, so that the above-mentioned properties of the remote unit 14 mainly determine the size of the remote coverage areas 10.
The equipment in the DAS 12 in FIG. 1 may be provided to support wide radio bands of spectrum commonly used in the cellular industry. For example, a personal communications services (PCS) band may be supported by the DAS 12 that includes the 1850-1910 MegaHertz (MHz) radio band for uplink signals and 1930-1990 MHz band for downlink signals. A cellular radio band may also be supported by the DAS 12 that includes the 824-859 MHz radio band for uplink signals and the 869-894 MHz band for downlink communications signals. In this regard, it may be required to couple a base station to a DAS, such as DAS 12 in FIG. 1, through a duplexed port. A duplexed port allows a DAS to simultaneously receive downlink communications signals into the DAS and transmit uplink communications signals from the DAS.
In this regard, FIG. 2 illustrates exemplary downlink and uplink path circuits 28D, 28U provided in respective downlink and uplink communications paths 30D, 30U in the DAS 12 of FIG. 1. The downlink and uplink communications paths 30D, 30U extend between the base station 18 and a plurality of remote units 14(1)-14(M). The base station 18 is coupled to the DAS 12 via a duplexed port 32. The duplexed port 32 receives downlink communications signals 20D from the base station 18 to be provided to the DAS 12 via the HEE 16 in this example. The duplexed port 32 also receives uplink communications signals 20U from the DAS 12 via the HEE 16 to be provided to the base station 18. A head-end duplexer 34(H) is provided in the HEE 16. The head-end duplexer 34(H) is coupled to the duplexed port 32. The head-end duplexer 34(H) is configured to separate a duplexed downlink and uplink communications path 36 into the separate downlink communications path 30D and a separate uplink communications path 30U. Downlink communications signals 20D are coupled from the head-end duplexer 34(H) to the head-end downlink circuits 28D(H). The downlink communications signals 20D are then distributed from the head-end downlink circuits 28D(H) to the remote downlink circuits 28D(R) in each remote unit 14 to be transmitted through the antenna 24 of the remote unit 14. The uplink communications signals 20U are coupled from the antenna 24 of the remote unit 14 to a remote duplexer 34(R), and from the remote duplexer 34(R) to the remote uplink circuits 28U(R). The uplink communications signals 20U are distributed to the head-end uplink circuits 28U(H), and from the head-end duplexer 34(H) to the base station duplexed port 38.
With continuing reference to FIG. 2, due to expansion of radio bands, the frequency gap between downlink communications signals 20D and the uplink communications signals 20U supported in the DAS 12 may become smaller. For example, the frequency gap between the downlink communications signals 20D and the uplink communications signals 20U may be 10 MHz or less. If a frequency gap between the downlink communications signals 20D and the uplink communications signals 20U is too small, it may be difficult or even impossible to provide the required isolation between the downlink and uplink communications paths 30D, 30U in each remote duplexer 34(R) while maintaining other requirements of the remote duplexer 34(R), such as low attenuation, lower ripple (i.e., variance in frequency response), small size, and/or low cost. If the isolation provided by each remote duplexer 34(R) is lower than required, a portion of the downlink communications signal 20D can leak through the remote duplexer 34(R) to the uplink communications path 30U, as shown by leakage path 40 in FIG. 2. This leakage through the uplink communications path 30U might distort the uplink communications signal 20U or even create oscillations on the uplink communications signal 20U.